Gas-liquid contactors are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, such that their emission into the atmosphere is closely regulated by clean air statutes. The method by which such gases are removed with a spray tower or other type of gas-liquid contactor is known as wet flue gas desulfurization.
The cleansing action produced by a gas-liquid contactor is generally derived from the passage of gas upwardly through a tower countercurrently to a descending liquid which cleans the air. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia-based solutions. As used herein, a slurry is a mixture of solids and liquid in which the solids content can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates which can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases which are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry. Thereafter, the slurry can be accumulated in a tank.
A known type of gas-liquid contactor is a spray tower 10, an example of which is shown in cross-section in FIG. 1. The spray tower 10 generally is an upright structure composed of a tower 14 equipped with an inlet duct 12 through which combustion gases enter the tower 14. The inlet duct 12, as well as other appropriate sections of the tower 14, are preferably formed from a high nickel alloy so as to promote their corrosion resistance. Above the inlet duct 12 is a lower bank of spray headers 16 which introduce a spray 20 of a cleansing liquid, often an alkaline slurry, into the tower 14. A second, upper bank of spray headers 18 is typically provided above the lower bank of spray headers 16, with additional banks of spray headers being used as may required for a given application. One or more pumps 26 are required to recycle the cleansing liquid by pumping the liquid from a tank 30 to the banks of spray headers 16 and 18. Each bank of spray headers 16 and 18 may be individually equipped with a pump 26 for the purpose of promoting the flexibility of the pumping and spraying operation to accommodate varying demands by the scrubbing operation.
Intimate contact between the liquid spray 20 and the flue gases rising through the tower 14 results in a cleansing action, by which the liquid and the entrapped or reacted gases are collected at the bottom of the tower 14 in the tank 30. The cleansed gases which continue to rise through the tower 14 then typically pass through a mist eliminator 22, and thereafter are either heated or passed directly to the atmosphere through a chimney 24.
Due to structural considerations, conventionally-accepted design practices typically limit the width of the inlet duct 12 to about 2/3 of the diameter of the tower 14. In addition, the first bank of spray headers 16 must typically be about six to about ten feet (about 2 to about 3 meters) above the inlet duct 12, so as to provide a suitable volume for gas-liquid mass transfer time, during which gases are absorbed by the liquid, and to prevent the liquid spray 20 from entering the inlet duct 12, which would otherwise create a slurry and particulate buildup requiring periodic removal. Conventional practices also typically limit the flue gas velocity within the inlet duct 12 to about fifty to about sixty feet per second (about fifteen to about eighteen meters per second) for the purpose of maintaining an acceptable pressure drop and gas distribution within the tower 14. The above limitations generally dictate both the height of the inlet duct 12 and the position of the first bank of spray headers 16 relative to the inlet duct 12.
In view of the above, it can be appreciated that the tower diameter, the height of the inlet duct 12, and the distance of the first bank of spray headers 16 above the inlet duct 12 must all be increased in order to accommodate increased flue gas flows through the spray tower 10. Consequently, the overall size and height of the spray tower 10 is dependent on the amount of flue gases to be scrubbed. In turn, taller spray towers 10 necessitate more powerful pumps 26 to pump the liquid to the spray headers 16 and 18, whose vertical height must also increase so as to position the spray headers 16 and 18 sufficiently above the inlet duct 12.
Those skilled in the art will appreciate that, in view of the considerations noted above, it would be desirable to minimize the height of a flue gas spray tower for the purpose of minimizing construction, operational and maintenance costs of the tower and the scrubbing operation.